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Headshot of Emily Lakdawalla

Finally! New Horizons has a second target

Posted by Emily Lakdawalla

15-10-2014 13:02 CDT

Topics: trans-neptunian objects, New Horizons, pretty pictures, New Horizons KBO target, mission status

What a huge relief: there is finally a place for New Horizons to visit beyond Pluto. A team of researchers led by John Spencer has discovered three possible targets, all in the Cold Classical part of the Kuiper belt. New Horizons will most likely visit one named "PT1" for "[New Horizons] potential target 1." PT1 has been imaged four more times by Hubble since its discovery, and those followup images have provided enough information on its orbit for four independently working teams to determine that New Horizons will be able to fly close past it in January 2019. It is probably about 30-45 kilometers in diameter and is easily reachable with New Horizons' limited fuel budget; targeting it will require only about 35% of the spacecraft's remaining fuel. New Horizons will have to fly an additional billion kilometers beyond Pluto in order to reach it where it orbits 43.4 AU away from the Sun.

New Horizons at its Kuiper belt target

Alex Parker, SwRI

New Horizons at its Kuiper belt target

This discovery has been a long time in coming. New Horizons launched toward Pluto in 2006, with plans for a further flyby of a second, likely much smaller Kuiper belt object. The Kuiper belt phase of the mission will turn New Horizons from a single flyby mission into a Kuiper belt tour. The only catch: the second target had not been discovered yet. Based on the current understanding of the population of the Kuiper belt, the New Horizons team expected that a focused survey with large telescopes would yield one or a few objects within New Horizons' reach. Unfortunately, what years of ground-based surveys actually yielded was the unpleasant discovery that our prior understanding of the Kuiper belt population was wrong. There were fewer small objects than predicted, and no reachable object was discovered. This summer, a desperate New Horizons team argued for, and won, the opportunity to use the Hubble Space Telescope to search. I wrote about the Hubble search for a New Horizons Kuiper belt target at length back in June.

Hubble has rescued the New Horizons Kuiper belt mission. In fact, PT1 was discovered during the pilot program, the small initial survey designed to validate the larger search, in pictures taken just a week after the search began. The whole search turned up a total of five potential targets; two have been ruled out. PT1 is certainly targetable, while PT2 and PT3 are potentially targetable.

Three potentially targetable Kuiper belt object for New Horizons


Three potentially targetable Kuiper belt object for New Horizons
From June to August 2014, the Hubble Space telescope searched a region of the sky to find small Kuiper belt objects that New Horizons could reach after its Pluto encounter. The search yielded five objects, and Hubble performed followup observations from August to October. Those observations ruled out two, but confirmed that one in particular is 100% certain to be reachable by New Horizons.

It was a huge team effort, John Spencer told me. He said "Marc Buie led the data reduction effort, and was the first to find nearly all our Kuiper belt objects. Alex Parker was also a key player, working on predicted orbits that allowed us to stack the data correctly and on orbit determination - it was he who first determined that one of our objects was 100% targetable. Larry Wasserman and Yanping Guo helped us to confirm targetability. Hal Weaver played a huge role in the proposal and in the observation design, and Alan Stern was of course closely involved in all stages of the work. Simon Porter, Amanda Zangari, Anne Verbiscer, Susan Benecchi, Ray Sterner, and Dave Borncamp put a lot of effort into searching the data, and JJ Kavelaars, Keith Noll, Mark Showalter, Jean-Marc Petit, Cesar Fuentes, Dave Tholen, and Mike Belton contributed to the proposal and general strategizing."

PT1 was discovered on June 27 -- just 11 days after the search team was awarded time on Hubble -- in a photo taken by Hubble on June 26. Its existence was flagged by an automated processing pipeline and confirmed by eye by Marc Buie the same day. I asked Alan Stern who would get the discovery credit, and he said it would go to the entire team.

Discovery images of New Horizons' Kuiper Belt target

NASA, ESA, SwRI, JHU/APL, and the New Horizons KBO Search Team

Discovery images of New Horizons' Kuiper Belt target
Five Hubble Space Telescope Wide Field Camera 3 images of a faint, distant object that will be visited by New Horizons in January 2019. The photos were taken on June 24, 2014. The object has a magnitude of 26.8, too faint to be observed by ground-based telescopes.

Hubble took followup photos on August 2, 3, 21, and 23 to determine its orbit, and on August 22 Alex Parker determined that the object was targetable by New Horizons. Since then, four independent analyses have confirmed it's within New Horizons' reach. It's a lot of data to sift through; so far, Hubble has acquired 830 images with its Wide Field Camera 3 in its search pattern, plus 100 more follow-up photos of all the objects discovered so far. And it's not easy to spot the faint objects they are looking for. The images contain a million stars that are brighter than PT1.

How close will New Horizons get? Initially, Alex told me, they couldn't entirely rule out the possibility that New Horizons would impact it. The likelihood of that was vanishingly tiny, of course, and after taking the followup images the uncertainty in PT1's position decreased to the point that New Horizons' path no longer intersected the range of the object's possible future positions. But the proximity means that the New Horizons team will be able to choose arbitrarily how close they want to fly to the object, limited by the uncertainty in their understanding of its orbital path. Picking that distance will require balancing the desire to get high-resolution observations with engineering constraints like how fast the spacecraft can rotate at closest approach to target the object. If the object is 30 kilometers in diameter, New Horizons' highest resolution camera, LORRI, would get 100 pixels across it at a range of 60,000 kilometers, or 1000 pixels across it at a range of 6000 kilometers. They will target the object in a burn well after the Pluto encounter, between October and December of 2015.

Orbits of Pluto and PT1, New Horizons' flyby targets

Alex Parker

Orbits of Pluto and PT1, New Horizons' flyby targets
This diagram shows the orbits of the planets (blue), Pluto (white), and New Horizons' Kuiper belt target PT1 (orange), as well as the path of New Horizons (yellow). The diagram also contains dots for other cold classical Kuiper belt objects (orange dots) as well as asteroids and other Kuiper belt objects (white). The Kuiper belt dots are from a model population, based on Canada-France Ecliptic Plane Survey results. A few large Kuiper belt objects are called out in their real locations with large white dots: Eris, Makemake, and Haumea. A labeled still from this animation is also available.

The object does not yet have a formal name; it is called "1110113Y" on the Hubble website and "PT1" within the New Horizons team. Another numerical designation will come after the object is submitted to the Minor Planet Center, which the search team says it will do after they perform followup observations with Hubble in October to pin down the astrometry more precisely. Eventually, it will get a provisional name (2014 followed by some letters and numbers) from the Minor Planet Center. Hopefully it will get a formal name before January 2019, which is when New Horizons will fly past it.

Although PT1 is the most likely of the Hubble-discovered objects to be targeted, it's possible that followup observations may make PT2 or PT3 more desirable. PT2 and PT3 are both slightly brighter than PT1 and are therefore probably larger. Unfortunately, it's not possible to target two of these objects within New Horizons' fuel budget; they must select one.

What do we know about PT1 so far? Its orbit is circular and close to the plane of the ecliptic, so it is a Cold Classical Kuiper belt object, meaning that it has had a very different history from Pluto. Pluto is a member of a population of objects in the Kuiper belt whose orbits were changed as Neptune migrated outward, scattering them. Pluto now has an inclined and elliptical orbit that is locked in a resonance with Neptune, such that Pluto orbits the Sun twice for every three times Neptune does. In contrast, Cold Classical objects were probably never tossed around in this way. So PT1 could be very pristine, a cold, never-heated relic of solar system formation. On the other hand, it's very small, estimated to be 30 to 45 kilometers in diameter, and scientists think that most objects of that size are not primordial, but are actually fragments from collisions of larger objects, which would make it less pristine. No matter what, its size and orbital position mean that it will look very, very different from Pluto.

PT1, the potential target for New Horizons after Pluto

NASA, ESA, and G. Bacon (STScI)

PT1, the potential target for New Horizons after Pluto
PT1 is a small, likely lumpy object, only 30-45 kilometers across. The view takes in nearly the entire solar system, with the Sun and the four giant planets appearing as bright stars.

What will it look like? We don't know. Here is a similar-sized world, Saturn's moon Helene, which is about 36 by 32 by 30 kilometers in diameter. But Helene is likely not a good analog, because its surface is affected by the large amount of dust wandering around the Saturnian system.

Helene in enhanced color

NASA / JPL / SSI / color composite by Gordan Ugarkovic

Helene in enhanced color
Cassini flew within 7,000 kilometers of Helene, Rhea's leading co-orbital satellite, on June 18, 2011. The Saturn-facing hemisphere of the moon is covered with strange gully-like features that probably represent slides of dry material into local topographic lows.

Here's another similar-sized world, Ida, which is 56 by 24 by 21 kilometers in diameter. Ida is probably a collisional fragment. But it's made mostly of rock, not like the icier Kuiper belt objects. So it's probably not a good analog either. We do know that a large fraction of Kuiper belt objects have satellites; it's certainly possible that PT1 does.

Ida and Dactyl in color

NASA / JPL / SSI / color composite by Emily Lakdawalla

Ida and Dactyl in color
Galileo took the three images for this enhanced-color view of asteroid 143 Ida and its satellite Dactyl on August 28, 1993 at about 16:30 UTC, about 20 minutes before its closest approach. Galileo was about 16,000 kilometers from the asteroid at the time

But there's no reason to expect that PT1 will look like neither of these. Just like Pluto and Charon, PT1 is a world the likes of which we've never visited before. We'll have to wait until 2019 to find out what a small cold classical Kuiper belt object looks like!

See other posts from October 2014


Or read more blog entries about: trans-neptunian objects, New Horizons, pretty pictures, New Horizons KBO target, mission status


suitti: 10/15/2014 01:23 CDT

I love that the name 1110113Y is almost binary. Maybe it's a contact binary... Most humor is binary. Either it's funny or it isn't.

Jonathan Ursin: 10/15/2014 02:42 CDT

Cool! My prediction, only a prediction feel free to agree or disagree. Without the experience a heavy bombardment or much solar weathering this world will appear like a fresh boulder or iceberg full of sharp edges and unusual features.

Quayley: 10/15/2014 02:49 CDT

Great blog, saw the nasa news release, this is so much more in depth, beautiful visualisation of Kuiper belt targets too. Any spectra of these objects? I know they were hoping for a white one to compare the more reddish pluto

lepton: 10/15/2014 04:08 CDT

It is wonderful news! Looking forward for 2015 and 2019 flybys. If we haven't wasted money on ISS, we would probably have orbiters/landers for all the planets and major moons now.

Tim R: 10/15/2014 05:13 CDT

The Parker graphic of the Kuiper belt and orbits is great. Article, too. The size of PT1 has significant advantages. First consider what recently arrived in a small package - comet 67P. PT1 is 10 times larger in size, 1000x in volume. However, PT1's size means that it has likely not been differentiated by its gravity, that is, little or no layering of its interior. That is a big difference between PT1 and Pluto and its larger moons and a valuable one. Another thing that PT1 will offer is a, as you point out - pristine KBO that can be compared to the physical properties of the irregular Saturn moons. Its likely that several Saturn moons are KBOs and New Horizons flybys will begin to help better categorize and identify Saturn captured KBOs. The quick discovery of so many KBOs by HST in such a short span and discovery KBOs of such small size is amazing. HST cannot commit significant time to surveying the vastness of the Kuper Belt. Note that the search summary diagram can be used by anyone to estimate how many KBOs HST could find and probably could also arrive at an estimae of the number of KBOs total with a couple of assumptions. The thought it also raises is that the donated Hubble-class telescopes could be committed to searching the Kuiper Belt. KBOs are now and will become an even hotter topic of research area. I think a lot of grad students will flock to it. The next gen of detectors will improve our studies of KBOs markedly. Sending a low-cost probe to Enceladus could be better justified by emphasizing the value of the irregular moons.

rboerner: 10/15/2014 05:32 CDT

Once New Horizon's second target has been chosen, and the post-Pluto burn executed, will a new search be performed to find perhaps a third KBO at an even greater distance that would be reachable after flyby #2? We would have three years for that search.

Fred Thurber: 10/15/2014 09:30 CDT

I seem to remember that the New Horizon RTG did not get a full load of plutonium because of a shortage before launch thus potentially shortening the mission. Will it have enough power in 2019?

TheBigH: 10/15/2014 09:41 CDT

This is excellent news. Finding a third, post-PT1, object to study should be a priority.

Mark: 10/16/2014 02:08 CDT

Another excellent piece Emily. It seems I get all my detailed planetary exploration news from your blogs these days. Your level of research and the quality of your writing is amazing. Thank you so much for keeping us all informed.

ernestborg9: 10/16/2014 08:48 CDT

Lepton, the money spent on ISS was NOT a waste.

Lorient: 10/17/2014 08:13 CDT

Very, very good news and an excellent post as usual!

Michael Welford: 10/17/2014 01:14 CDT

Next comes the Sedna orbital zone. Can NH last however many decades it takes for an encounter? Will it be able to send back useful information? Will NASA pay for such an extended mission? Is it too soon to book time for a target search on the Webb telescope?

Emily Lakdawalla: 10/17/2014 02:54 CDT

It would be nice for there to be a third target, but I wouldn't hold my breath. They do still have more work to do on the Hubble data to see if there are any fainter targets still lurking in there. As for Webb -- the problem is that the longer it takes to discover a potential target, the harder it will be to reach, because you get more bang for your molecule of propellant the sooner you do the trajectory correction maneuver to reach the target. It's not impossible that they'll find another one, by any means; but it's not looking likely. Regarding the plutonium: I forget how long NH will last, but I'm pretty confident it's until at least 2020, and I'm sure that (as they did with the Voyagers) they can cut back on science instruments to keep running longer, if NASA chooses to.

Torbjörn Larsson: 10/18/2014 09:48 CDT

I'm fascinated by that the cold KBO PT1 @ 43.4 AU lies well within the hot KBO Pluto aphelion @ 48.9 AU. (Which by the way means PT1 is 43.4 - 32.9 ~ 10 AU further out after the NH encounter with Pluto.) We will get "the best of two worlds". In fact, the still image makes it seem like PT1 orbits close to the main density of the Kuiper Belt, which is further interesting. (And also means the likelihood of a 3d target diminishes. :-/)

Torbjörn Larsson: 10/18/2014 09:52 CDT

@lepton: ISS is a science lab (and is used for technology development), so it is not "wasted money". And perhaps the commercial LEO business that it has spawned will mean further gains.

Lindsay: 10/20/2014 01:39 CDT

As expressed by Tim R the discovery of these small objects at such vast distances is truly mind blowing and will indeed result in some fascinating statistics emerging over the coming years on the relative size abundance of the Kuiper belt population. With the prospect of larger telescopes coming online in the 2020’s most notably the LSST, the Kuiper belt may well be mapped out to a high precision fairly soon. Any future discoveries along New Horizons path will probably come too late for the probe due mainly to spacecraft longevity as alluded to by Emily but you never know. Perhaps they will provide the stimulus for a successor outer planet - multiple KBO flyby mission. Just need to find a few that string out in a line over time! What I find even more astounding than the actual detection of the small target bodies is the implication that the probe will be able to navigate precisely enough for a meaningful flyby. Can we really expect to be able to see surface features? The majority of any targeting would have to be done on the basis of Earth based observations in order to get NH close enough to even detect the object with LORRI. PT1 is perhaps only marginally larger than Pluto’s two smallest moons. I am not sure that New Horizons can currently detect those while some 8 months out from the Pluto encounter. It follows that any New Horizons based navigation to target PT1 will only be at the later stages of the approach. I suppose we live in an era of atmospheric blur cancelling telescopes and an increase in precision over what went before. Even so with a target now in our sights it will be a remarkable achievement to deliver this incredibly far off flyby.

lodaya: 10/28/2014 09:18 CDT

Having exhausted a lot of fuel, could one use a last blast to turn NH around and make it fall back into the Sun? Carefully avoiding all bodies we know of, but making sure one passes by something interesting like Neptune, or a second pass past Pluto. Or is that really implausible?

Rob: 02/21/2015 06:42 CST

PT1 (or the final decisive target) should officially be named Tombaugh. Hopefully this can get through the bureaucracy. Can't wait to see Binary Planet Pluto up close!!!!!

Marshall Eubanks: 06/27/2015 01:09 CDT

Did they ever announce which one got selected? I did a search, and found nothing. It occurred to me that maybe the Webb Telescope, once launched, could find another candidate or two for a _third_ flyby, even further out in the solar system. (A third candidate would be even dimmer than these, and HST probably couldn't see it just as Earth-bound telescopes can't see these.)

Marshall Eubanks : 07/22/2015 11:25 CDT

They have gotten more permanent names: "PT1" = 2014 MU69 and "PT3" = 2014 PN70 I talked to Alan Stern about the possibility of having a 3rd flyby. That would require picking PT1 (as it would take less fuel to reach) and, of course, finding something even further out, possibly a Sednoid outside the Kuiper Belt per se. The PT1 / PT3 target selection will apparently be later this summer.

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